Device for inductively heating semiconductor material



April 1, 1969 w. KELLER ETAL 3,435,509

DEVICE FOR INDUC TIVELY HEATING SEMICONDUCTOR MATERIAL Filed July '7.1966 c SW CK CH/: C I" United States Patent 3,436,509 DEVICE FORINDUCTIVELY HEATING SEMICONDUCTOR MATERIAL Wolfgang Keller, Pretzfeld,and Otto Schmidt, Erlangen, Germany, assignors to SiemensAktiengesellschaft, Erlangen, Germany Filed July 7, 1966, Ser. No.563,504 Claims priority, applicat9i0n Germany, July 9, 1965, S 8 88 0Int. Cl. 1105b /04, 9/02 US. Cl. 21910.77 5 Claims ABSTRACT OF THEDISCLOSURE Our invention relates to a device for inductively heatingsemiconductor material.

Devices are known for inductively heating semiconductor material whereinthe coil of a tuned circuit, hereinafter referred to as the heatingcircuit, is employed as an induction heating coil, the heating circuitbeing coupled to an oscillating circuit, or so-called tank circuit,which is connected in the anode circuit of a transmitting tube. In orderto achieve maximum power transfer or transmission, the coupling is madeas strong as possible. The generator frequency is tuned to one of theslopes or limbs of the resonance curve of the heating circuit so that itis possible to control the heating power by varying the generatorfrequency or, on the other hand, to adjust it at a constant value byemploying suitable automatic controlling means. Reference can be had toUS. Patent No. 3,046,379 of W. Keller et al. and US. Patent No.3,265,470 of .W. Keller for further detail-s of devices of the foregoingtypes.

Difliculties have arisen due to the fact that disturbances in theheating circuit such as, for example fiashover between the windings ofthe coil or between the coil and the semiconductor material cause thegenerator frequency to leap across to the other slope or limb of theresonance curve whereby for the most part decrease in heating powertakes place. In such case however the heating power becomes changed bythe automatic controlling device which further reduces it and theheating action is discontinued. In addition to the fact thatreadjustment of the generator frequency by the operating personnelconsequently becomes necessary, the semiconductor rod which has beenheated now becomes unusable due to the discontinuance of the heatingaction.

It is accordingly an object of our invention to provide a device forinductive heating of semiconductor material which will avoid theforegoing disadvantages of the known devices and the necessity forreadjusting the generator frequency, and will minimize the production ofunusable semiconductor rods.

It is a further object of our invention to provide a device forinductive heating of semiconductor material which includes a tankcircuit and a heating circuit of such construction and combination thatthe device will have a stable performance without any failure of theheating action during a zone melting operation.

More particularly in accordance with our invention we provide a heatingcircuit with as little as possible supercritical coupling to theoscillating or tank circuit.

With the foregoing and other objects in view, we provlde in accordancewith the invention a device for the inductive heating of semiconductormembers comprising a high-frequency generator having an oscillatingcircuit connected in the anode circuit of a transmitting or oscillatingtube, and a heating circuit at least critically coupled thereto, theheating circuit including an induction coil for surrounding thesemiconductor material.

Whereas in the known constructions, the high-frequency voltage isincreased at the tank circuit by connecting the anode of thetransmitting tube to a tap of the tank circuit coil so as to accordinglyrender the coupling therebetween weaker, the effectiveness of such aconstruction is nevertheless limited because, in the case of high powergenerators that are operated with a high anode voltage, due to anincreased high frequency voltage, fiashover can take place between thewindings of the tank circuit coil. Consequently, more specifically inaccordance with the invention, the capacitance of the heating circuit isset at a value which is at most 40 times larger than that of theoscillating or tank circuit. Such a ratio of the capacitance of theheating circuit to that of the tank circuit has been found to ensure acompletely stable performance.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a device for inductively heating semiconductor material, it isnevertheless not intended to be limited to the details shown, sincevarious modifications and structural changes may be made therein withoutdeparting from' the spirit of the invention and within the scope andrange of equivalents of the claims.

The construction and the method of operation of the invention, however,to gether with additional objects and advantages thereof will be bestunderstood from the following description of a specific embodiment whenread in connection with the accompanying single figure of the drawingwhich consists of a circuit diagram of the device.

A transmitting or oscillating tube R together with a tank circuitincluding a capacitor C and a coil L form a generator in delta or Yconnection. A heating circuit including a capacitance C and aninductance L are coupled alternately by a capacitor C and a coil L tothe tank circuit. A switch SW is adapted to be moved between twopositions in one of which the capacitor C becomes the coupling agent forthe heating circuit and in the other of which the coil L becomes thecoupling agent for the 'heating circuit. The inductance L consists of acoil which is adapted to surround the semiconductor material such as arod S which is to be heated in a zone melting operation, for example.With a generator frequency of 1.5 mHz., the device of our invention hasthe following dimensions and component values:

The heating coil consists of three windings having a diameter of about30 mm. and an inductance of about 0.5 microhenry. The heating circuitcapacitance therefore has a value of 18,000 picofarads (pf.). Theminimum value of the tank circuit capacitance depends upon the ratio C/C =40, so that the tank circuit capacitance is thus 450 pf. when theheating circuit capacitance is 18,000 pf. As a practical example, a tankcircuit capacitance of 1,000 pf. Was selected so that stable performancewas thereby ensured. The capacitance of the coupling C is 600 pf., forexample. If the coupling is to be effected by induction, the switch SWis shifted from its position in the drawing to a position in which theinductance L is connected in the circuit. The inductance L can consistof a coil having 10 windings with a diameter of mm. When the generatorfrequency is located at the capacitor limb or slope of the resonancecurve of the heating circuit, it is advantageous to couple the heatingcircuit with the tank circuit by means of the coupling capacitor C asshown in the figure. In this case, the formation of idle or reactancecurrents in the cable between the tank circuit and the heating circuitcan be avoided. This will also apply to the case where coupling iseflected by the inductance L when the generator frequency is located onthe inductive slope or limb of the resonance curve of the heatingcircuit.

We claim:

1. Device for inductively heating semiconductor material, comprising ahigh-frequency generator having a tank circuit including a capacitanceconnected in the anode circuit of a transmitting tube, and a heatingcircuit at least critically coupled to said oscillating circuit, saidheating circuit including a coil adapted to surround the semiconductormaterial for heating the same inductively, and having a capacitance upto forty times greater than the capacitance of said tank circuit.

2. Device according to claim 1, wherein said generator is tuned to afrequency located on one limb of the resonance curve for said heatingcircuit.

3. Device according to claim 2, wherein said tuned frequency is locatedon the capacitive limb of said heating circuit resonance curve, and saidheating circuit is coupled capacitively to said oscillating circuit.

4. Device according to claim 2, wherein said tuned frequency is locatedon the inductive limb of said heating circuit resonance curve, and saidheating circuit is coupled inductively to said oscillating circuit.

5. Device for inductively heating semiconductor material comprising ahigh-frequency generator having an oscillating circuit including a firstcapacitance and a coil connected in the anode circuit of a transmittingtube, a heating circuit including a second coil adapted to surround thesemiconductor material for heating the same inductively and a secondcapacitance up to forty times greater than said first capacitance ofsaid oscillating circuit, and coupling means comprising a thirdcapacitance, a third coil and a switch actuatable for coupling saidheating circuit and said oscillating circuit to one another alternatelythrough said third capacitance and said third coil.

References Cited UNITED STATES PATENTS 2,773,192 12/1956 Blok et a1.219--10.77 X 3,265,470 8/1966 Keller 21910.77 X

RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner.

